Due to the increasing larger number of stories of buildings, the greater sophistication of building design technology, etc., fire-resistant designs were reevaluated in Japan as a project of the Ministry of Construction. The “New Fire-Resistant Design Law” was enacted in March 1987 as a result. Due to this, the limitation on fire-resistant coverings requiring that the temperature of the steel materials at the time of fires be kept to no more than 350° C. was reassessed. It became possible to select the suitable method of fire-resistant covering from the relationship between the high temperature strength of the steel material and the actual load of the building. For this reason, when it is possible to secure a high temperature strength satisfying the design standard of 600° C., that is, by using a steel material with a high temperature strength of 600° C., it became possible to simplify or reduce the fire-resistant covering.
To deal with this trend, steel materials for building use having a predetermined strength even when the building becomes on fire etc. and becomes a high temperature, which is so-called fire-resistant steel, is being developed. Here, fire-resistant steel envisioning a temperature of the building at the time of a fire of 600° C. and able to maintain strength at that temperature will be discussed.
As the strengthening mechanisms for obtaining high temperature strength at 600° C. of steel materials, the four types of mechanisms of (1) increased fineness of the crystal grain size of the ferrite, (2) dispersion strengthening by a hard phase, (3) precipitation strengthening by fine precipitates, and (4) solid-solution strengthening by alloy elements are well known.
(1) Increased fineness of crystal grain size of ferrite: Dislocations moving in the grains move to adjoining crystal grains through the crystal grain boundaries (below, also called the “grain boundaries”), so the crystal grain boundaries act as resistance to movement of dislocations. Therefore, if the crystal grains become fine, the frequency of the dislocations crossing the crystal grain boundaries when moving becomes higher and the resistance to movement of dislocations increases. The strengthening method using the increased fineness of the ferrite crystal grains to increase the resistance to movement of dislocations drops in effect due to grain growth at a high temperature. For this reason, in fire-resistant steel, the strengthening method using the increased fineness of the ferrite crystal grains is seldom used alone.
(2) Dispersion strengthening by hard phase: In a hard phase, compared with a soft phase, dislocations have a hard time moving in the crystal grains and the resistance required for deformation is large. Therefore, in a macro structure comprised of a hard phase and soft phase mixed together (called a “double phase structure”), the increase in the volume percentage of the hard phase causes a rise in strength. For example, in a double phase structure comprised of ferrite and pearlite, if the volume percentage of the hard phase of pearlite rises, the strength increases. However, this method has the problem of an easy drop of toughness due to the hard phase.
(3) Precipitation strengthening by fine precipitates: Precipitates distributed on the sliding surfaces act as resistance to movement of dislocations in the crystal grains. In particular, fine precipitates are effective in strengthening at a high temperature, so conventional fire-resistant steels often utilize this precipitation strengthening. In particular, in conventional fire-resistant steels, Mo is added to cause the formation of fine Mo carbides and improve the high temperature strength by precipitation strengthening (for example, see Japanese Patent Publication (A) No. 5-186847, Japanese Patent Publication (A) No 7-300618, Japanese Patent Publication (A) No. 9-241789, and Japanese Patent Publication (A) No. 2005-272854). In these conventional fire-resistant steels, the amount of C is made about 0.1% and Mo is made to precipitate as Mo carbides without becoming solid-solute. In addition, a steel material utilizing the fine precipitation of Cu to improve the high temperature strength has also been proposed (for example, see Japanese Patent Publication (A) No. 2002-115022).
However, in precipitation strengthening, in general, the problem is known that the base material falls in toughness and the weld heat affected zone at the time of welding (called the “HAZ”) also falls in toughness due to the precipitates coarsened by the effect of the heating.
(4) Solid-solution strengthening by alloy elements: The alloy elements solid-solute in the steel (called “solid solution alloy elements”) have elastic stress sites formed around them, so are dragged by dislocations and become resistances to movement of the dislocations. This is referred to as “drag resistance”. Its magnitude is affected by the misfit of the solid solution alloy elements and the steel, that is, the difference in sizes of the solute atoms and solvent atoms, the concentration and diffusion coefficient of the solute atoms, etc. Note that the effect of solid solution alloy elements being dragged by dislocations and generating drag resistance is referred to as the “drag effect”.
Solid-solution strengthening utilizing this drag effect is starting to be studied as a strengthening mechanism of fire-resistant steel. To utilize this solid-solution strengthening, it is necessary to reduce the carbon, nitrogen, etc. and inhibit the formation of carbides, nitrides, and other precipitates. For example, Japanese Patent Publication (A) No. 2006-249467 proposes a fire resistant steel material utilizing Mo as a solid solution alloy element. In this fire resistant steel material, Mo and B (boron) are included to raise the hardenability, while the upper limit of Mn is made 0.5% or lower than the general amount of addition to avoid excessive rise in strength.
Further, fire-resistant steel is also being proposed by Japanese Patent Publication (A) No. 5-222484, Japanese Patent Publication (A) No. 10-176237, Japanese Patent Publication (A) No. 2000-54061, Japanese Patent Publication (A) No. 2000-248335, Japanese Patent Publication (A) No. 2000-282167, etc. However, the fire-resistant steels in these references cover hot rolled steel plates with thin plate thicknesses etc. and do not consider the toughness of the base material and weld heat affected zone and the high temperature ductility of the weld heat affected zone required in thick-gauge steel plates, H-beams, and other thick-gauge steel materials. For this reason, there are the problems that:
a) To inhibit the precipitation of nitrides of Nb, Ti is added in excess. In thick-gauge steel materials, coarse Ti precipitates are formed and the toughness of the base material and weld heat affected zone cannot be secured,
b) Al is added in excess for deoxidation, so in thick-gauge steel materials, the drop in toughness due to island-shaped martensite becomes a problem,
c) B (boron) is sometimes included, so measures cannot be taken against the drop in high temperature ductility of the weld heat affected zone, that is, reheating embrittlement, etc.